Tissue dissector apparatus

ABSTRACT

Surgical apparatus and method includes a cannula that houses an endoscope and supports a dilating element near a distal end of the cannula. The dilating element has a dimension which is greater than the diameter of the cannula for enlarging a surgical cavity in tissue as the cannula is advanced through tissue at a surgical site to provide working space adjacent a target vessel within which surgical instruments may be conveniently manipulated. The dilating element of oval sided shape permits surrounding tissue to be pushed away or otherwise displaced away from the target vessel atraumatically. A locking mechanism is disposed on the cannula, which accepts a succession of mating dilating elements of progressively larger dimensions for successive insertion and enlargement of a surgical cavity as required. In one embodiment, the dilating element is made of rigid plastic, and in another embodiment, the dilating element is made of resilient material that may be confined within a retractable sheath which, in the extended position, encases and compresses the dilating element to a smaller dimension and which, in a retracted position, allows the dilating element to resiliently expand and enlarge the surgical cavity.

This is a continuation of application Ser. No. 09/133,136 filed on Aug.12, 1998, now abandoned, which is incorporated by reference herein inits entirety.

FIELD OF THE INVENTION

The present invention relates to the field of surgical apparatus, andmore particularly to endoscopic vessel isolators.

BACKGROUND OF THE INVENTION

During surgical harvesting of vessels, a target vessel is exposed,tributaries are ligated and transected, and the vessel is harvested. Inorder to view the vessel, a cannula housing an endoscope is insertedinto a surgical cavity to visualize the adventitial layer of a targetvessel. The vessel is tracked by advancing the cannula along the path ofthe vessel while bluntly dissecting the cavity as the cannula isadvanced. Upon viewing a side branch or tributary of the vessel, asurgical tool is inserted into the surgical cavity to cauterize andsever the side branch. The endoscope remains in the surgical cavityduring this process to allow the surgeon to view the procedure, and thesize of the cavity is maintained using insufflating gas. Using differenttools simultaneously in a surgical cavity is difficult due to the smallsize of the surgical cavity. Additionally, within the surgical cavity,the surrounding tissue typically collapses upon the cannula and surgicaltools, increasing the difficulty of the operation, if performed withoutinsufflation. However, maintaining the surgical cavity open usinginsufflation with gas under pressure then also requires slidinggas-tight seals for each endoscopic instrument that is inserted into thesurgical cavity.

Current systems commonly employ a balloon coupled to the cannula forintermittent inflation and deflation to enlarge the surgical cavity asthe cannula is advanced. However, use of a balloon to enlarge surgicalcavities has the disadvantage that multiple balloon inflation anddeflation tires the surgeon's hands, and makes it difficult to retainthe precise hand control needed to perform the surgical procedure. Also,manufacture of a balloon cannula requires manual mounting of the balloonin a tedious process that adds expense to the device. Additionally,balloons have a potential for rupture during use and thereby disrupt thesurgical procedure. Thus, a device is needed which retains theendoscopic vessel tracking ability of current systems, while alsoenlarging the surgical cavity without the disadvantages of balloonsystems.

SUMMARY OF THE INVENTION

In accordance with the present invention, a tissue dissector is providedin which a cannula houses an endoscope, and a dilating element iscoupled near the distal end of the cannula. The dilating element has anouter dimension which is greater than the diameter of the distal end ofthe cannula. This greater dimension serves to enlarge the surgicalcavity as the cannula is advanced through the surgical site, thusallowing the cannula to track along the vessel while forming a workingcavity and providing room within which additional surgical tools mayoperate safely. In one embodiment, the dilating element is in the shapeof an oval, allowing compression of the surrounding tissue to occuratraumatically.

In an alternate embodiment, a locking mechanism is disposed on thecannula, and the dilating element is coupled to the locking mechanismwhen enlargement of the surgical cavity is required. In this embodiment,multiple dilating elements of differing outer dimensions may be employedresponsive to the enlargement required. Various locking mechanisms maybe employed in accordance with the current invention, including usingscrew threads disposed on the surface of the cannula, and matinginternal screw threads in a bore hole through the dilating element topermit the dilating element to couple to the screw threads.Alternatively, the dilating element may include a bayonet-type fitting,with mating knobs on the associated surface portion of the cannula forlocking the dilating element into place. Additionally, in one embodimentthe tip and dilating element are a single detachable component, and maybe coupled and decoupled to the main body of the cannula as desired.This greatly facilitates use of dilating elements of differentdimensions.

The body of the cannula may be tapered from a smaller diameter near thedistal end of the cannula to a larger diameter remote from the distalend of the cannula. The tip of the cannula is transparent to facilitateendoscopic viewing of the surgical cavity. The tapering of the distalend of the cannula may begin at a point forward of the distal end of thedilating element. This allows the tip of the cannula to track along thetarget vessel without the enlarged diameter of the dilating elementpreventing the tip from making contact with the target vessel. In oneembodiment, the dilating element is made of rigid plastic to facilitateexpansion of a working cavity and ease of translation through thesurgical site. In another embodiment, the dilating element is made of aflexible material which compresses as the external walls exert forceupon the cannula, but retains sufficient structural rigidity toaccomplish the required enlargement of a working cavity. In yet anotherembodiment, the dilating element is made of flexible material and isshrouded within a retractable sheath which, in the extended position,encases the dilating element and thereby compresses the dilating elementto a smaller diameter, and in a retracted position, allows the dilatingelement to expand and enlarge the working cavity.

Methods are also disclosed for dissecting an elongated cavity along thecourse of a vessel using a cannula according to one or other embodimentsof the present invention, including incising the skin of a patient,placing the tip of the cannula along the surface of the vessel,advancing the cannula along the vessel under continuous endoscopicvisualization through the tip, enlarging the cavity about the outerdimension of the dilating element, removing the cannula upon reachingthe desired length of target vessel, and optionally placing a sealingtrocar in the incision and maintaining the enlargement by insufflatingthe subcutaneous tunnel with gas under pressure. The vessel may then beharvested through a separate incision near the remote end of thesurgical cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of the cannula in accordance withone embodiment of the present invention.

FIG. 2 is a cut-away side sectional view of the cannula in accordancewith the present invention.

FIG. 3 is a perspective view of the distal end of cannula in accordancewith the present invention.

FIG. 4 is a flow chart illustrating the application of cannula inaccordance with the present invention.

FIG. 5 a illustrates a cannula having a locking mechanism in accordancewith one embodiment of the present invention.

FIG. 5 b is a side view of a dilating element for locking attachment tothe cannula in FIG. 5 a.

FIG. 5 c is a cut-away side sectional view of the dilating element foruse with the cannula of FIG. 5 a.

FIG. 6 a is a cannula having an alternate embodiment of a lockingmechanism in accordance with the present invention.

FIG. 6 b is a side view of a dilating element for locking attachment tothe cannula of FIG. 6 a.

FIG. 6 c is a cut-away side sectional view of the dilating element foruse with the cannula of FIG. 6 a.

FIG. 7 is an exploded view illustrating the removable module of tip anddilating element on a cannula in accordance with another embodiment ofthe present invention.

FIG. 8 is a flow chart illustrating the operation of the interchangeabledilating element embodiment of the cannula in accordance with thepresent invention.

FIG. 9 a is a cut-away side sectional view of an embodiment of thepresent invention including a retractable sheath illustrated in extendedposition.

FIG. 9 b is a cut-away side sectional view of the embodiment of FIG. 9 awith the retractable sheath illustrated in retracted position.

FIG. 10 is a flow chart illustrating the operation of the retractablesheath embodiment of the cannula in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a tissue dissector 50 in which a cannula 100 iscoupled to a dilating element 112. The proximal end of cannula 100 iscoupled to a handle 116 and the distal end of cannula 100 is enclosed bytransparent tapered tip 104. Dilating element 112 is positioned inwardlyfrom the distal end of the cannula 100. Cannula 100 may be made from avariety or combination of bioinert, substantially inelastic materials,such as stainless steel, polyethylene, polyurethane, polyvinyl chloride,polyimide plastic, and the like that preferably have a tensile strengthof at least 10,000 psi. Handle 116 is ergonomically formed to allow asurgeon to easily and comfortably manipulate cannula 100 within asurgical cavity.

FIG. 2 illustrates a cut-away side sectional view of tissue detector 50.As shown, the distal end of cannula 100 has an outer diameter ordimension 136, and the dilating element 112 has an outer dimension 132which is greater than the diameter 136. The proximal portion of cannula100 preferably has a smaller dimension than the dilating element 112 toallow more flexibility in maneuvering the cannula 100 in the surgicalsite. The greater dimension 132 of dilating element 112 enlarges orexpands a surgical cavity by pushing away surrounding tissue within asurgical cavity as the cannula 100 is advanced through a surgical site.The surgical cavity may thus be formed adjacent to a target vessel as aresult of the blunt tissue dissection caused by the tapered tip 104 asit is advanced along the path of a target vessel, such as the saphenousvein. A preferable diameter 136 of cannula 100 is about 8.5 mm, andpreferable outer dimensions 132 of dilating elements 112 are in therange from about 15 mm to about 30 mm. Thus, in application, a surgicalcavity is initially formed by the tapered tip 104, and is initiallyincreased or enlarged to the diameter 136 of the distal end of cannula100. Additionally, in accordance with the present invention, thesurgical cavity is further enlarged by the dilating element 112substantially to the dimension 132 of the dilating element 112 and thislatter enlargement or expansion of a surgical cavity constitutes two orthree times greater enlargement than the enlargement of such a surgicalcavity by the diameter 136 of the distal end of the cannula 100. Thisenhanced enlargement of a surgical cavity eases further dissection oftissue as the cannula 100 is advanced along a target vessel, andfacilitates subsequent manipulation of surgical tools within suchsurgical cavity.

Cannula 100 houses an endoscope 120 for viewing the surgical site andthe target vessel through the transparent tip 104. The proximal end ofendoscope 120 is attached to the proximal end of the cannula 100 bymating screw threads 128 at the proximal end of the cannula 100 and theproximal end of the endoscope 120 for fixedly positioning the endoscope120 within the cannula 100. The proximal end of the endoscope 120 mayinclude an eyepiece or camera attachment, or the like (not shown), andthe distal end of endoscope 120 is positioned near the distal end of thecannula 100 in alignment with the tip 104 for visualization therethrough of tissue being bluntly dissected thereby as the cannula 100 isadvanced along a target vessel.

Referring to FIG. 3, tip 104 is made of a transparent material, such aspolycarbonate plastic. Positioning endoscope 120 near the distal end ofcannula 100 and in alignment with the transparent tip 104 thereforeallows a surgeon to view objects forward of the cannula 100. Thisenables the surgeon to advance the cannula 100 along the path of atarget vessel, and to view and thereby avoid avulsing any side branches.Tip 104 is tapered from the distal end thereof (that is blunted with aradius of about 0.045 inches) to the larger diameter of the proximal endof tip 104 that is approximately the diameter 136 of the distal end ofthe cannula 100. Tapering of the tip 104 over a taper length of about0.500 to about 0.800 inches allows advancement of the cannula along thevessel without excessive force and injury to the vessel, as well asbetter visualization via the endoscope 120 of a target vessel throughthe tapered walls of the transparent tip 104.

In order to track the path of a target vessel effectively, the taperedwall of tip 104 is placed against the target vessel as the cannula 100is advanced through connective tissue. The taper angle 116 of the tip104 allows the target vessel to be seen more clearly and allows a lengthof vessel equivalent to the length of the taper of the tip 104 to beseen by the surgeon. In order to enable the tapered wall of tip 104 tolay against the target vessel, a spacer length 108 of cannula 100between the dilating element 112 and the proximal end of tip 104 isprovided to set the dilating element 112 back behind the taper angle 116of the tapered wall of tip 104. This spacer length 108 of cannula 100may have a diameter substantially equal to the outer diameter 136 of thedistal end of the cannula 100. The spacer length prevents dilatingelement 112 from interfering with the contacting of the target vessel bythe walls of the tapered tip 104, at taper angle 116. Without anintervening spacer length 108, the dilating element 112 more closelyadjacent the tip 104 would prevent the tapered wall of tip 104 fromcontacting the target vessel within the taper angle 116, and this wouldincrease the force exerted on the target vessel during cannulaadvancement. In one embodiment, the distal end of dilating element 112is 14-28 mm from the proximal end of the tip 104. Cannula 100 ispreferably about 32-47 cm long, and tip 104 is preferably about 10-15 mmlong.

Dilating element 112 is preferably formed of Teflon or polyurethane, orpolycarbonate, or the like, to form a rigid shape which compresses orotherwise displaces tissue on the walls of the surgical cavity to forman enlarged surgical cavity. In an alternate embodiment, dilatingelement 112 comprises resilient foam which compresses in response to anapplied external force. For example, pressure from inserting thedilating element 112 into a small incision may reduce the diameter ofthe dilating element 112 and prevent the dilating element 112 fromcausing further rupture or tearing of the incision. Since the tissuetypically surrounding a target vessel such as the saphenous vein is softfatty tissue, a foam dilating element 112 with sufficient resilience andrigidity may push back the fatty tissue and enlarge a surgical cavityadjacent the vessel. Dilating element 112 is preferably of oval shape tofacilitate atraumatic expansion of the surrounding tissue followingblunt dissection of the fatty tissue by the tapered tip 104. Of course,other shapes of dilating element 112 may be used that have maximumdimensions 132 greater than the dimension of the proximal end of tip104.

In application, as shown in FIG. 4, the surgeon incises 400 the skin ofthe patient and dissects 404 to expose the surface of the target vessel.The surgeon next places 408 the tapered wall of the transparent tip 104on the surface of the vessel and advances 412 the tip 104 and cannula100 under endoscopic visualization through the tip 104 along the path ofthe target vessel. Following dissection of the cavity along the vessel,the cannula 100 is removed 416, and a sealing trocar may be placed 420in the incision for insufflating 424 the subcutaneous tunnel with gasunder pressure to maintain the enlargement of the cavity. The vesselthus isolated is then harvested 428. A combined endoscopic anddissection instrument may be introduced through the sealing trocar toligate and remove the target vessel for use, for example, as a coronaryartery or peripheral vascular bypass graft. Alternatively, the isolatedvessel may be left in place for surgical formation of an in-situfemoropopliteal or femoral-distal graft. Alternatively, followingincision of the skin of the patient and dissection to expose the surfaceof the target vessel, gas insufflation may be initiated through asealing trocar. The sealing trocar may be loaded onto the shaft of thecannula 100 prior to fixation of the dilating element 112 (if the outerdimension 132 of the dilating element 112 is greater than the innerdiameter of the sealing trocar). The advancement 412 of the cannula 100may then be conducted under gas insufflation, to improve visualizationof the previously formed surgical cavity.

FIG. 5 a illustrates an embodiment of cannula 100 with a lockingmechanism 150 for a detachable dilating element 112 as shown in FIG. 5b. Locking mechanism 150 includes a length of screw threads disposed onthe surface or outer housing of the cannula 100 at a position near thedistal end of the cannula that allows the locked dilating element 112 tobe located in a position on the cannula 100 as described previouslyherein with reference to FIGS. 1-3.

FIG. 5 c illustrates a cross-section of the dilating element 112 havinga mating lock or set of screw threads 162 which couples to lockingmechanism 150 of FIG. 5 a. A bore hole 154 is formed along thehorizontal axis of the dilating element 112 and the screw threads aredisposed along a portion of the bore hole 154 as a mating lock 162. Thedimension 158 of the bore hole 154 is wider than the diameter 136 of thecannula 100 but is small enough to ensure a tight coupling uponinserting the dilating element 112 into the locking mechanism 150. Thus,in this configuration, the dilating element 112 is locked onto thecannula 100 by rotating the grooved end of the dilating element 112around the shaft of the cannula 100 until the distal end of the screwthreads 150 on the cannula 100 contacts the unthreaded portion of thedilating element 112 in the bore hole 154. At this point, the dilatingelement 112 is locked.

FIG. 6 a illustrates an alternate embodiment of locking mechanism 150. Aknob or protuberance is disposed on the surface of cannula 100 formating with a corresponding groove 162, as shown in FIG. 6 c, in thedilating element 112. The groove 162 is formed to slide over knob 150and mate therewith through partial rotation on the cannula 100 forlocking the dilating element 112 of FIG. 6 b in place. In anotherembodiment, the locking mechanism 150 includes the groove in the surfaceof the cannula 100, and the dilating element 112 includes theprotuberance disposed in the bore hole of the dilating element 112.

FIG. 7 illustrates an exploded view of an embodiment of cannula 100 inwhich the tip 104 and the dilating element 112 are fixably coupledtogether as a unit, and are detachable from the distal end of cannula100. This embodiment allows convenient change of dilating elements 112by simply removing the dilating element 112 and tip 104 unit forreplacement with an alternate dilating element 112 of differentdimension and tip 104 unit. Threads 170 positioned at the distal end ofthe cannula 100 allows a threaded bore hole 154 (not shown) in thedilating element 112 or tip 104 unit to couple to the threaded shaft170. This embodiment employing detachable or interchangeable dilatingelements 112 allows the surgeon to control the size of the surgicalcavity being dissected in tissue. This is accomplished by couplingdilating elements 112 of differing outer dimensions 132 to the cannula100 which, in turn, enlarge the surgical cavity to sizes correspondingsubstantially to the dimensions 132 of the dilating elements 112.Different surgical cavities require different amounts of enlargement andtherefore the surgeon may select the amount of enlargement provided bythe cannula 100 in a specific surgical cavity in accordance with themultiple dilating elements 112 that may be attached and utilized insuccession in accordance with the described embodiments of the presentinvention.

The flow chart of FIG. 8 illustrates a method for isolating a targetvessel using the detachable dilating element 112. The surgeon incises800 the skin and dissects 804 to expose the adventitial surface of thetarget vessel. The surgeon next places 808 the transparent tapered tip104 on the adventitial surface of the vessel. The cannula 100 isadvanced 812 under endoscopic visualization through the tip 104 untilthe target vessel is sufficiently isolated. The cannula 100 is removed816 after establishing a subcutaneous tunnel or surgical cavity adjacentthe target vessel which is more constricted than desirable, andtherefore requires greater enlargement. The dilating element 112 is thenremoved and replaced 820 with a larger dilating element 112 and thecannula 100 is again advanced 824 through the tunnel until the surgicalcavity is sufficiently dissected using interchangeable dilating elements112 in a succession of progressively larger dimensions as necessary toattain the required amount of enlargement of the surgical cavity. Thecannula 100 is removed 828 and a sealing trocar is placed 832 in theincision and the tunnel is insufflated 836 with gas under pressure tofacilitate harvesting the vessel 840.

The cut-away side sectional views of FIGS. 9 a and 9 b illustrate analternate embodiment of cannula 100 in which a slidable sheath 160 isemployed to reduce the outer dimension 132 of the dilating element 112.In this embodiment, the dilating element 112 includes resilientlycompressible foam, as described above. The sheath may be formed as aplastic tube which is slidably disposed on the cannula 100 and which hasa distal end 168 and a proximal end 172.

Upon sliding or extending the sheath 160 in a distal direction, thedistal end 168 of the sheath 160 encases the dilating element 112 andthereby compresses the dilating element 112 to a reduced dimension 132.Upon retracting the sheath 160 by sliding the sheath 160 in a proximaldirection, the distal end 168 of the sheath 160 releases the dilatingelement 112 which resiliently expands to a larger dimension 132, asshown in FIG. 9 b. Thus, by compressing the dilating element 112 uponinserting the cannula 100 into an incision, rupture or tearing of theincision is minimized. When properly placed, the sheath 160 is retractedto enable resilient expansion of the dilating element 112, thereby toprovide enlargement of the surgical cavity.

In application, as shown in the flow chart of FIG. 10, the surgeonincises 1000 the skin and dissects 1004 to expose the adventitialsurface of the target vessel. The surgeon next extends 1006 the sheathand places 1008 the transparent tapered tip 104 of the cannula 100, withthe sheath extended, on the surface of the vessel. The sheath 160retains the dilating element 112 in compressed configuration as thecannula is advanced 1012 until the ensheathed dilating element 112 is inselected position under the skin. The sheath 160 is retracted 1016 toallow the compressible dilating element 112 to expand. The cannula 100is advanced 1020 under endoscopic visualization through the tip 104until the target vessel is sufficiently isolated. The cannula 100 isremoved 1024, and a sealing trocar is placed 1028 in the incision andthe tunnel is insufflated 1032 with gas under pressure to facilitateharvesting 1036 the isolated vessel.

Therefore the method and apparatus of the present invention facilitateenlargement of a surgical cavity simultaneous with the advancement ofthe cannula 100 through the surgical cavity, without requiringintermittent manual manipulation of balloons or other similar devices.Additionally, the method and apparatus of the present invention providesfor dilating the surgical cavity to different dimensions responsive tointerchanging detachable dilating elements 112. Finally, the method andapparatus of the present invention provides for a dilating element 112which has a compressible resilient dimension for insertion through anincision in a state of compressed dimension for minimizing rupture ortearing of the incision while still providing for enlargement of thesurgical cavity in a state of resilient expansion.

1. A tissue dissector, comprising: an elongated cannula having aproximal end and a distal end and a central axis extending therebetween;a distal tip having tapered outer walls converging to a blunt end fordissecting tissue, the tip being disposed on the distal end of thecannula to dissect tissue and facilitate passage of the cannula throughtissue; a length of screw threads positioned on an outer surface of thecannula proximal to the distal tip; and a dilating element disposed onthe cannula proximal to the distal tip, the dilating element having asmooth oval shaped exterior contour symmetrically disposed about thecentral axis to facilitate atraumatic expansion of tissue followingdissection by the tapered distal tip advancing through tissue, thedilating element having a cross-sectional dimension of the oval shapethat is greater than a cross-sectional dimension of the distal end ofthe cannula and greater than a cross-sectional dimension of the distaltip, the dilating element further comprising a threaded bore hole formedin the dilating element for engaging the length of screw threads on thecannula for removably positioning the dilating element on the cannula.2. The tissue dissector of claim 1, where in the dilating element issolid.
 3. The tissue dissector of claim 1, further including a spacerlength of cannula of between 14-28 mm disposed between a distal end ofthe dilating element and a proximal end of the distal tip.
 4. The tissuedissector of claim 1, wherein the cross-sectional dimension of the ovalshape of the dilating element is at least two times larger than thecross-section sectional dimension of the distal end of the cannula. 5.The tissue dissector of claim 4, wherein the cross-sectional dimensionof the dilating element is between 15-30 mm.
 6. The tissue dissector ofclaim 1, wherein the exterior contour of the dilating element is anoval-shape.
 7. The tissue dissector of claim 1, wherein the dilatingelement is compressible.
 8. A tissue dissector kit, comprising: anelongated cannula having a proximal end and a distal end; a distal tiphaving tapered outer walls converging to a blunt end for dissectingtissue, the tip being disposed on the distal end of the cannula todissect tissue and facilitate passage of the cannula through tissue; alocking mechanism positioned on the cannula proximal to the distal tip;and a plurality of dilating elements each adapted to mount on thecannula proximal to the distal tip, each dilating element having asmooth oval shaped exterior contour to facilitate atraumatic expansionof tissue following dissection by the tapered distal tip, each dilatingelement having a cross-sectional dimension of the oval shape that isgreater than a cross-sectional dimension of the distal tip, thecross-sectional dimension of each dilating element being different fromone another, each dilating element further comprising a mating lockadapted to mate with the locking mechanism on the cannula for removablypositioning each dilating element on the cannula, wherein the differentdilating elements may be mounted one at a time on the cannula fordissecting tissue and therefore forming cavities of differingdimensions.
 9. The tissue dissector of claim 8, wherein the lockingmechanism comprises a length of screw threads positioned on an outersurface of the cannula, and the mating lock comprises a threaded borehole formed in each dilating element for engaging the length of screwthreads.
 10. The tissue dissector of claim 8, further including a spacerlength of cannula of between 14-28 mm disposed between a distal end ofeach mounted dilating element and a proximal end of the distal tip. 11.The tissue dissector of claim 8, wherein the cross-sectional dimensionof the oval shape of each dilating element is at least two times largerthan the cross-sectional dimension of the distal end of the cannula. 12.The tissue dissector of claim 11, wherein the cross-sectional dimensionof the oval shape of each dilating element is between 15-30 mm.
 13. Atissue dissector, comprising: an elongated cannula having a proximal endand a distal end; a distal tip having tapered outer walls converging toa blunt end for dissecting tissue, the tip being disposed on the distalend of the cannula to dissect tissue and facilitate passage of thecannula through tissue; and a solid dilating element of fixed outerdimension removably mounted on the cannula proximal to the distal tip,the dilating element having a smooth oval shaped exterior contour tofacilitate atraumatic expansion of tissue following dissection by thetapered distal tip, the dilating element having a cross-sectionaldimension of the oval shape that is greater than a cross-sectionaldimension of the distal end of the cannula and greater than across-sectional dimension of the distal tip, the cannula furtherincluding a length of screw threads positioned on an outer surface ofthe cannula proximal to the distal tip, and wherein the dilating elementfurther comprises a threaded bore hole for engaging the length of screwthreads and removably positioning the dilating element on the cannula.14. A tissue dissector, comprising: an elongated cannula having aproximal end and a distal end; and a dilating unit removably mounted onthe cannula distal end, including: a distal tip having tapered outerwalls converging to a blunt end for dissecting tissue, the tip beingdisposed on the distal end of the dilating unit to dissect tissue andfacilitate passage of the cannula through tissue; and a dilating elementhaving an oval shape of cross-sectional dimension greater than across-sectional dimension of the distal end of the cannula and greaterthan a cross-sectional dimension of the distal tip, the dilating elementbeing located proximally with respect to the distal tip to facilitateexpansion of tissue following dissection by the tapered distal tippassing through tissue, the cannula further including a length of screwthreads positioned on an outer surface of the cannula near the distalend thereof, and wherein the dilating unit further comprises a threadedbore hole for engaging the length of screw threads and mounting thedilating unit on the distal end of the cannula.